1. Field of the Invention
The present invention relates to a cathode ray tube apparatus having a cathode ray tube, which is used in an information terminal device for displaying characters and graphics and, more particularly, the present invention is directed to a cathode ray tube apparatus in which a beam landing drift and an image drift distortion which is caused by the influence of terrestrial magnetism is corrected.
2. Description of the Related Art
In a cathode ray tube apparatus, such as a television receiver or display apparatus having a color cathode ray tube (referred to hereinafter as "CRT" when necessary) with a color selection mask, such as an aperture grille or shadow mask, it is known that beam landing and image distortion are affected by terrestrial magnetism.
FIGS. 1 to 3 of the accompanying drawings show measured results of beam landing drift caused by terrestrial magnetism.
FIG. 1 shows measured results of drifts of landing patterns 1, 2 and 3 obtained when a face plate 10 of a cathode ray tube is faced to the direction East (E), the direction South (S), the direction West (W) and the direction North (N), respectively. Dotted line patterns in FIG. 1 depict reference line patterns obtained when there is no terrestrial magnetism, i.e., when there is no external magnetic field. Solid line patterns in FIG. 1 depict actual patterns changed by the terrestrial magnetism. When the face plate 10 of the cathode ray tube is faced to the direction East (E) and the direction West (W), the center patterns are placed at the same position in which the reference line pattern 1 is obtained in the absence of the terrestrial magnetism and the practical pattern 1 overlap each other.
FIG. 2 shows a beam landing drift amount .DELTA. obtained between a particular color beam 4 shown by a solid line and a beam 5 shown by a dashed line when the beam 4 is displaced in the direction shown by an arrow A. Specifically, the beams 4 and 5 bombards a fluorescent substance 8 of a particular color coated on the face plate 10 through an opening (slit or hole) in a color selection mask 6. In order to obtain a satisfactory color purity, it is an indispensable condition that a particular color beam, e.g., a green beam 4 should bombard the fluorescent substance 8 of this particular color.
FIG. 3 is a diagram showing plotted results of the beam landing drift obtained at six points (see FIG. 1) of the picture screen end portion when the cathode ray tube is rotated one time within the horizontal plane in the magnetic field caused by terrestrial magnetism under the condition that the tube axis of the cathode ray tube is laid in the horizontal direction. Study of FIG. 3 reveals that a beam landing drift amount .DELTA. is regularly changed in a sine wave fashion. In FIG. 3, the beam landing drift amount .DELTA. to the right-hand side as seen from the front surface of the face plate 10 is defined as a positive drift amount +.DELTA., and the beam landing drift amount .DELTA. to the left-hand side is defined as a negative drift amount -.DELTA..
FIG. 4 shows measured results of the changes of image distortion patterns obtained when image distortion patterns are changed by terrestrial magnetism. Specifically, FIG. 4 shows the changes of image distortion patterns obtained when the face plate 10 of the cathode ray tube is faced to the direction East (E), the direction South (S), the direction West (W) and the direction North (N), respectively. Dotted line patterns in FIG. 4 represent reference image distortion patterns obtained in the absence of terrestrial magnetism, i.e., when there is no external magnetic field. Solid line image distortion patterns in FIG. 4 represent practical image distortion patterns obtained when the image distortion is changed by terrestrial magnetism.
The beam landing drift and the change of the image distortion patterns due to the terrestrial magnetism become factors which deteriorate characteristics, such as color purity and pattern distortion of the cathode ray tube apparatus.
In order to reduce the factors under which characteristics are deteriorated by terrestrial magnetism, there have been proposed the following three techniques:
(1) reducing the terrestrial magnetic field with a magnetic shield (magnetic shield plate);
(2) reducing the terrestrial magnetic field with a degauss coil; and
(3) reducing the terrestrial magnetic field with a correction coil.
The above three techniques (1) to (3) will be described below, respectively.
(1) Reduction technique based on a magnetic shield:
As a reduction technique based on a magnetic shield, there are known CRT external magnetic shields and CRT internal magnetic shields. According to the magnetic shield technique, a magnetic field generated by terrestrial magnetism is weakened so that the beam landing drift amount and the changed amount of the image distortion can be reduced.
(2) Reduction technique based on a degauss coil:
The reduction technique based on a degauss coil is a technique in combination with the (1) reduction technique on which uses magnetic shield. According to this reduction technique, a degauss coil is attached to the tube side wall of the CRT and the degauss coil is supplied with an AC attenuation current. The magnetic shield and the color selection mask are thereby degaussed. Thus, the electron beam proceeds on its desired path to thereby reduce the influence of terrestrial magnetism.
(3) Reduction technique based on a correction coil:
The reduction technique based on a correction coil has hitherto been applied to a picture tube of a television receiver having a wide picture screen of about 25-inch or greater with a small beam landing allowance and a high-definition display tube. Japanese laid-open patent publication No. 4-61590 published on Feb. 27, 1992, for example, describes such a reduction technique based on a correction coil.
FIG. 5 is a schematic diagram showing a front arrangement of a cathode ray tube to which the reduction technique based on a correction coil is applied.
FIG. 6 is a schematic block diagram showing a fundamental arrangement of a correction circuit applied to the example shown in FIG. 5.
As shown in FIG. 5 and, as seen from the face plate 10 side of the cathode ray tube, 6 correction coils LCC-LT (landing correction coil left top), LCC-CT (landing correction coil center top), LCC-RT (landing correction coil right top), LCC-LB (landing correction coil left bottom), LCC-CB (landing correction coil center bottom) and LCC-RB (landing correction coil right bottom) are disposed at designated positions around the face plate 10 side.
As shown in FIG. 6, the correction coils LCC-LT, LCC-CT, LCC-RT, LCC-LB, LCC-CB and LCC-RB are driven based on a direction correction signal S1 output from a direction correction signal generator 21, a beam current correction signal S2 output from a beam current correction signal generator 22 and a local correction signal S3 output from a local correction signal generator 23 through a landing correction coil (LCC) driver 24.
The direction correction signal generator 21 generates the direction correction signal S1 which is a current signal corresponding to a direction code designated by a direction code switch 25 disposed on the panel surface of the cathode ray tube apparatus and supplies the same to the respective direction correction coils LCC-LT, LCC-CT, LCC-RT, LCC-LB, LCC-CB and LCC
FIG. 7 shows the contents of the direction correction signal S1. Waveforms in FIG. 14 are previously stored in the direction correction signal generator 21 in response to terrestrial magnetism drifts shown in FIG. 3.
Referring back to FIG. 6, the beam current correction signal generator 22 is supplied with an automatic brightness limit (ABL) signal S4 which is a signal having a level corresponding to a beam current from a terminal 26. Then, the beam current correction signal generator 22 time-integrates the ABL signal S4 to provide the beam current correction signal S2 used to correct color displacement caused by a thermal expansion of the color selection mask and supplies the beam current correction signal S2 to the respective direction correction coils LCC-T, LCC-CT, LCC-RT, LCC-LB, LCC-CB and LCC-RB.
The local correction signal generator 23 supplies the local correction signal S3 used to carry out the landing correction peculiar to the CRT and to the respective direction correction coils LCC-LT, LCC-CT, LCC-RT, LCC-LB, LCC-CB and LCC-RB.
However, the reduction technique (1) which employs a magnetic shield encountered with the following disadvantages:
It is impractical to shield the whole of the CRT, particularly, the entire consumer CRT with an ideal magnetic material, such as permalloy or the like from a financial standpoint. Therefore, it is customary that the CRT is only partly shielded. As a result, problems of unsatisfactory beam landing and the change of image distortion caused by an imperfect magnetic shield cannot be solved perfectly. Also, there is then the problem that the weight of the cathode ray tube is increased when such a magnetic shield material is used.
The reduction technique (2) based on a degauss coil has the following disadvantages:
Although improvement can be enhanced by increasing a magnetomotive force only the degauss coil, the improvement is of about half at maximum. There is then the problem that the degree of improvement is saturated and limited. Moreover, a degauss coil for providing a large magnetomotive force requires a large amount of copper so that the degauss coil becomes large in size, expensive and heavy.
Further, the reduction technique (3) based on a correction coil has the following disadvantages:
This reduction technique is effective only in correcting the beam landing drift but it cannot correct the image distortion drift at all. Moreover, the direction correction coils LCC-LT, LCC-CT, LCC-RT, LCC-LB, LCC-CB, LCC-RB and the direction correction coil driver 24 for driving the correction coils LCC-LT, LCC-CT, LCC-RT, LCC-LB, LCC-CB and LCC-RB are large in scale. There is also the problem that the cathode ray tube apparatus becomes expensive, heavy and complicated.